Electrical Power from a Turbine

The electrical power generated from a hydropower system could be estimated by the formula (details here):

E-Power = Head x ρ x Flow x Gravity x η

Where:
E-Power
Electrical power measured in W (Watts).
Head
For still water, this is the difference in height (m) between the inlet and outlet surfaces. Moving water has an additional component added to account for the kinetic energy of the flow. Total head equals the pressure plus velocity heads.

ρ

Density of water in (kg/m3).
Flow
Water flowing through the turbine (m3/s).
Gravity
Equivalent to 9.8 m/s2 (will vary according the latitude, longitide and altitude).
η
No turbine will capture all the potential energy contained in the falling water. There will also be mechanical losses in the rotor and electrical losses in the generator. The overall efficiency, is often called the ‘water to wire’ efficiency and has values typically between 60-70% for small hydro schemes, and is somewhat higher for larger schemes.

The Annual Energy Capture

To work out a gross estimation of the "marginal system value", the annual energy capture could be estimated. The amount of energy that can be produced each year at a given site is related to the design output of a turbine by the ‘Load Factor’. This is the relationship between the actual energy produced in a year and the maximum that could be produced if the plant were able to generate at full capacity for all 8760 hours in the year.

River Flow Data

Annual Energy Capture = Installed Capacity x Load Factor x 8760

For a typical run-of-river hydropower arrange the load factor is likely to be between 50% and 70%.

Simulated Example: With a height diference of 8 meters between one point and other 500 meters ahead on an irrigation channel minus losses height by infraestructure assume a 7 meters head. The high flow of the channel is 1.5 m3/s or 1500 litres per second. Assuming a ‘water to wire’ efficiency of 70%, the electrical power of the turbine at design flow will be:

Electrical Power = 7 x 1000 x 1.5 x 9.8 x 0.7 = 72 kW

With a typical load factor of 60% this would give an annual energy capture of:

Annual Energy Capture = 72 x 1000 x 0.6 x 8760 = 380 MWh

Here you could find many scenarios:

  • If this electricity was sold to the grid for $30/MWh this would result in a marginal system value of about $11,400 per year.
  • If your consumption of energy is 190 MWh at 10 cents/kWh and the balance was sold to the grid for 30 $/MWh this would result in a gross system value (save + marginal value) of about $24,700 per year.
  • If this electricity will replace energy produced with an oil generator at 35 cents/kWh this would result in a gross system value of about $133,700 per year.

Turbine Selection

Once is determined the potencial production of energy in the site one of the most important step is the selection of the turbine . In the small scale power plants, the main consideration to built a tailor made system is simplicity. The simplicity of the design and concept of the equipments pointed to obtain free or unexpert maintenance requirement and unattendant operation. You must obtain a new tool that help your develop activities and not a new facility that demand your attention. Turbines RangeThe Hydropower Systems use a water turbine to generate power. For Low-Head or Low-Flow schemes, the our main options of turbine are: Cross-flow, Francis, Kaplan and Pelton. There are other turbines configurations as propeller turbine, that has the advantage of low cost, but has high efficiency only when the flow is close to the design flow and hence will have very poor partflow efficiency and consequently a relatively low energy capture.

The Concept: Flowing water is directed on to the blades of a turbine runner, creating a force on the blades. Since the runner is spinning, the force acts through a distance (force acting through a distance is the definition of work). In this way, energy is transferred from the water flow to the turbine. Water turbines are divided into two groups; reaction turbines and impulse turbines. The precise shape of water turbine blades is a function of the supply pressure of water, and the type of impeller selected.

Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow. In reaction turbine pressure drop occurs in both fixed and moving blades as hapends in the Francis and Kaplan turbines..

Impulse Turbines: Impulse turbines change the velocity of a water jet. The jet impinges on the turbine's curved blades which change the direction of the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy. Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. In the Pelton and Crossflow turbines pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation.

 

CROSSFLOW TurbineCurve of efficiency of a Crossflow-turbine

A cross-flow turbine is drum-shaped and uses an elongated, rectangular-section nozzle directed against curved vanes on a cylindrically shaped runner. It resembles a "squirrel cage" blower. The cross-flow turbine allows the water to flow through the blades twice. The first pass is when the water flows from the outside of the blades to the inside; the second pass is from the inside back out. A guide vane at the entrance to the turbine directs the flow to a limited portion of the runner. The cross-flow was developed to accommodate larger water flows and low heads.Ossberger Crossflow turbine

With total efficiencies from 84% to 87%, in the curves of efficiency on your rigth, the advantages of a partially loaded Crossflow-turbine are illustrated. River flows are very small for several months of the year. During the months of small flow, the ability of a turbine to produce electricity depends on the characteristics of the course of efficiency of the relevant turbine. Turbines reaching high efficiency under normal conditions but rather low efficiency during small water flow reach a lower annual capacity in places with a variable water flow than turbines with a flat curve of efficiency.

Our Crossflow-turbines are made of individual standardized components configured according to customer requirements – i.e. according to the quantity of water and the head height of a particular location as a whole. Such a modular design enables all functions according to the project to be achieved at a good price.

Crossflow-turbines have a long service life, and are maintenance-free. During operation, they do not require any costly or complex spare parts; repairing them is feasible on site. A specific advantage of Crossflow-turbines is the possibility of using them in gravitation drinking water systems, even in very long conduits, not causing undesirable hydraulic impacts and thus not affecting the quality of drinking water during operation. This has been successfully tested several times in numerous countries around the world.

 

FRANCIS Turbine

Francis Turbine

 

 

A Francis turbine has a runner with fixed buckets (vanes), usually nine or more. Water is introduced just above the runner and all around it and then falls through, causing it to spin. Besides the runner, the other major components are the scroll case, wicket gates, and draft tube. The main caracteritics of design are:

  • Could be mounted on drinking water systems with long pipes and back pressure.
  • Minimum change in water flow through the turbine when transferring to the runaway speed = minimum hydraulic impacts.
  • Guide vanes and runner are made of stainless steel.
  • Electro-mechanical or hydraulic drive of the control with the option of full closing.
  • Special coating for humid environments (construction design for drinking water).
  • Special bypass structure for closed piping systems.
  • High efficiency at constant flow (50–100% of installed flow).
  • The runner is located directly on the generator shaft (alternatively with a clutch and own bearings of the turbine shaft).
  • Spiral casing, welded in segments.
  • Possible horizontal or vertical location of the shaft.

 

KAPLAN Turbine

A propeller turbine generally has a runner with three to six blades in which the water contacts all of the blades constantly. Picture a boat propeller running in a pipe. Through the pipe, the pressure is constant; if it isn't, the runner would be out of balance. The pitch of the blades may be fixed or adjustable, allowing for a wider range of operation.. The major components besides the runner are a scroll case, wicket gates, and a draft tube. Kaplan Turbine

Kaplan turbines with a flat efficiency curve can - without utilisation losses within the part load range - be designed for a large nominal flow. The final design could incorporate:

  • Most compact low-maintenance construction, that can be mounted easily.
  • Individual plan of installation of turbine.
  • Large flow.
  • Small civil requirements in case of new buildings.
  • Optimal with frontal admission and discharge.
  • For maximum flow in river power stations.
  • Direct-flow horizontal and pit-vertical water turbine.
  • The runner blades are made of bronze or stainless steel.
  • Excellent efficiency also achieved at partial load thanks to the use of two-level control.
  • Control drive by means of a high-pressure hydraulic unit.
  • Emergency closure of the guide apparatus by gravity, by closing weights.
  • The traverse mechanism closes the turbine to full, therefore, it is not necessary to install a closing plate.
  • Compact structure with minimized spatial requirements.
  • Simple installation thanks to the modular mode of the structure.
  • Design with a front gearbox and generator located in the pit or with flat belt drive.
  • Possible connection to the piping or to the concrete inlet.
  • The shaft sealing against water is maintenance-free thanks to ceramic packing.
  • The inlet- and drafttube casings are made of metal or cast concrete.

PELTON Turbine:

A Pelton turbine has one or more free jets discharging water into an aerated space and impinging on the buckets of a runner. Draft tubes are not required for impulse turbine since the runner must be located above the maximum tailwater to permit operation at atmospheric pressure.

Pelton Turbine

The water flow may be influenced through one or several needle jets that may be controlled finely. The water leaves the nozzles, hitting the subdivided runner blades tangentially.

The water jet is deflected in the hollows of the blades by almost 180 degrees, transmitting its energy to the turbine. So the water exclusively acts on the turbine runner through the deflecting pressure that is produced.

Special for high heads and small flows, could incorporate:

  • 1 or 2-nozzle horizontal and 1 up to 6-nozzle vertical arrangement
  • For small and very variable quantities of water
  • Nozzle diameters: 32, 50, 63 and 80mm
  • Direct connection of the runner to the generator shaft or the turbine shaft bearing
  • Hydraulically controllable coaxial or needle nozzles
  • High efficiency thanks to special geometry
  • Resistant to wear thanks to high resistance stainless steel (runner, nozzle body, jet and nozzle needle)
  • For long conduits and high pressure –design with a deflector
  • For drinking water systems –variant with electric control
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